feat: Add an export for validateCommand to allow other tools to reuse the validator's argument parsing

[nellh]

Exporting validateCommand lets OpenNeuro provide the validator arguments for any commands that requires validation (upload) without needing to duplicate the command parsing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (596b9e9) 85.58% compared to head (127e8c3) 83.57%.
Report is 55 commits behind head on master.

❗ Current head 127e8c3 differs from pull request most recent head 20c36b1. Consider uploading reports for the commit 20c36b1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1844      +/-   ##
==========================================
- Coverage   85.58%   83.57%   -2.01%     
==========================================
  Files         131       92      -39     
  Lines        6638     3897    -2741     
  Branches     1537     1273     -264     
==========================================
- Hits         5681     3257    -2424     
+ Misses        849      542     -307     
+ Partials      108       98      -10     

Files	Coverage Δ
bids-validator/validators/json/json.js	100.00% <100.00%> (ø)
bids-validator/validators/tsv/tsv.js	94.19% <ø> (ø)

... and 40 files with indirect coverage changes

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